I assume Speedtest servers, as they wouldn't have a way to get measurements for individual cell towers at scale.
(at least, I don't recall being able to get that sort of info from iOS APIs, nor have I ever seen data that would have required being derived that way)
If you search "5g latency", Google's AI answer says 1 ms, followed by another quote lift from Thales Group™ saying 4G was 20 ms and 5G is 1ms.
Once you scroll past the automated attempts, you start getting real info.
Actual data is in the "SpeedTest Award Report" PDF, retrieved from https://www.speedtest.net/awards/united_states/ via https://www.speedtest.net/awards/reports/2024/2024_UnitedSta....
Spoiler: 23 ms median for fastest provider, T-mobile.